Non-invasive measurement of skin impedance is described in the patent literature, for example, in U.S. Pat. No. 5,890,489 which issued on Apr. 6, 1999, and international patent application No. PCT/US 98/02037 published initially on Aug. 12, 1999 under WO 99/39627, and again as U.S. Pat. No. 6,517,482 on Feb. 11, 2003. These documents describe the use of skin impedance measurements in determining the level of a subject's blood glucose.
Any understanding of factors that influence the variability of impedance measurements in determining blood glucose levels that leads to an improvement in the reliability of such measurements is of inherent value, particularly to diabetics that need to continually monitor their blood glucose levels. It is well known that there is a need for a person to be able to quickly and conveniently make such a determination. There is evidence that increased frequency of blood glucose determination, which can lead to a finer level of control of blood glucose, can lead to an improved outcome, especially over a period of many years. With improved convenience and economics of blood glucose determination could thus enable a diabetic to improve outcome through increased frequency of monitoring of blood glucose levels.
In dermatology experiments and investigations, where many sites are used, the skin properties of the sites are often considered to be identical from one site to the next, or at least nearly so. This is evidently not always true. In fact, the properties of human skin can vary from place to place on the body (1). The properties also vary locally on body parts. Nicander et al. (2) measured skin impedance and trans-epidermal water loss (TEWL) (3, 4) at three locations on forearms: one close to the crook of the arm, one close to the wrist, and one in between, and found differences between the three sites. Moreover, Rodrigues and Pereira (5) showed that TEWL close to the crook of the arm and close to the wrist is higher than in between, where the TEWL is relatively constant. They also found that TEWL of the most active arm is higher than the accessory.
Since the baseline properties of the forearms vary locally, the skin of the forearms reacts differently from place to place to external stimuli. Van der Valk and Maibach (6) found significant irritation patterns along volar forearms and concluded that the potential irritation varies gradually between the wrist and crook of the arm. Similar results were demonstrated by Tur et al. (7). They studied the photoplethysmographic response of a drug on various locations at the arms and found differences between upper and lower part of the volar forearms, but not between right and left arm, or between inner and outer sides of the arms.